I'm not really bothered which is quicker - I'm guessing this is wifi anyway. I just want to know which would allow me to get the most life out of my iPhone 4S when tethering to it with may MBP. Thanks!

Although Bluetooth might use less instantaneous power, it will be slower and thus leave the radios turned on longer. Having the cellular radio turned on for data tethering will burn the most power by far, especially if you are far from the nearest cell tower.

I am writing this from a taxi cab between Baltimore and DC. Bluetooth connection between iPad and iPhone 4s giving me over 500kb speed up and down. Last hour use took less than 5% of the battery on either device. Wifi uses a lot more.